Far bias on the radial line bisection task: measuring perceptual-attentional and motor-intentional bias in normal subjects.

Normal subjects usually err distally when bisecting radial lines below eye level in midsagittal space (far bias). To learn if a radial line bisection bias may be characterized as primarily perceptual-attentional or primarily motor-intentional, we had subjects bisect lines while watching their hand and the line under two video monitoring conditions. In the DIRECT condition, proximal (near) and distal (far) on the video monitor corresponded with the workspace where subjects bisected lines. In the INDIRECT condition, we reversed proximal and distal as they appeared on the monitor. Thus, in the indirect condition, distal hand movement appeared proximal, and vice versa. In the DIRECT condition, subjects erred toward far space (mean 2.075 mm). In the INDIRECT condition, however, subjects erred proximally (mean 1.59 mm, near bias). Distal-proximal reversal of errors over the group of subjects is consistent with a primarily perceptual-attentional bias. In lower body space, normal visual-perceptual-attentional systems may be more biased toward far space than are kinesthetic-motor systems.